1. Field of the Invention
The present invention relates to calibration in a wireless communication system. More particularly, the present invention relates to a calibration apparatus and a calibration method for multicell Multiple Input Multiple Output (MIMO) transmission in a multiple antenna system.
2. Description of the Related Art
In a cellular wireless communication system, constant efforts are exerted to mitigate inter-cell interference. Recently, a method for reducing the interference more actively through cooperation between base stations is underway. In particular, cooperative Multiple Input Multiple Output (MIMO) schemes that extend various MIMO schemes, applied to a single cell, to a plurality of base stations are suggested. To use the cooperative MIMO scheme, the base stations require channel information of each mobile station. At this time, feedback of the necessary channel information is mostly in proportion to a performance of the cooperative MIMO scheme. To maximize the performance of the cooperative MIMO scheme, a considerable Channel State Information (CSI) needs to be fed back.
In a Time Division Duplex (TDD) system, the base stations may obtain the channel information using a sounding training signal without large-capacity data feedback. In addition, it is possible to greatly reduce overhead when using the sounding training signal with the cooperative MIMO scheme, as a plurality of the base stations may simultaneously obtain the channel information using the sounding training signal transmitted from the mobile station. Moreover, the use of the sounding by the base stations needs to be preceded by calibration, which synchronizes transmit and receive Radio Frequency (RF) chains between each individual base station and the mobile station.
The calibration process, by which reciprocity between an uplink channel and a downlink channel is obtained, compensates for amplitude and phase shifts of signals generating in the transmit RF chain and the receive RF chain of a transmitter and a receiver. Hence, the prior calibration is essential when the sounding is applied and various schemes are suggested for performing this process. However, conventional calibration methods target the transmitter and the receiver within the single cell and do not take account of the sounding for the cooperative MIMO scheme. As a result, even when the calibration is conducted per cell according to the conventional calibration method, calibration complex constants of the base stations involving the cooperative MIMO are different from each other and thus, the performance of the cooperative MIMO is degraded.
Therefore, a need exists for a calibration apparatus and a calibration method for multicell MIMO transmission based on uplink sounding in a multiple antenna system